Objective: The objective of this study was to compare the clinical utility of a new bioassay for thyrotropin (TSH) receptor antibodies (Abs) with the conventional radioreceptor assay and with measurement of thyroid peroxidase Abs in the diagnosis of Graves disease in childhood. Study design: Serum samples obtained from 22 children and adolescents with Graves disease (19 hyperthyroid, 3 in remission), 13 children and adolescents with chronic lymphocytic thyroiditis, and 17 normal children in a control group were evaluated. Results: TSH receptor Abs were detected by bioassay in 10 (91%) of 11 patients with active Graves disease but in 0 of 2 patients in remission, 0 of 13 normal members of the control group, and 0 of 11 patients with chronic lymphocytic thyroiditis including 1 with thyrotoxicosis. The sensitivity and specificity of TSH receptor Abs detected by radioreceptor assay studied in the same 11 patients and in an additional 11 patients was similar to bioassay. In contrast, thyroid peroxidase Abs were detected in only 12 (71%) of 17 patients with Graves disease but in 11 of 11 patients with chronic lymphocytic thyroiditis and in 0 of 17 members of the control group. Conclusion: Bioassay of TSH receptor Abs is both sensitive and specific for the diagnosis of active Graves disease in the young. When cost and simplicity are considered, however, bioassay offers no advantage over radioreceptor assay for initial diagnostic screening. Rather, bioassay for TSH receptor Abs may be useful in thyrotoxic patients who are negative initially in the radioreceptor assay or in treated patients whose clinical picture is discordant with results in the radioreceptor assay. (J Pediatr 1998;132:612-8)